A memorial service for the three New Zealand soldiers killed by a road-side bomb in Afghanistan will likely be held on Saturday, Prime Minister John Key says.

Corporal Luke Tamatea, 31, Lance Corporal Jacinda Baker, 26, and Private Richard Harris, 21 were killed when their Humvee hit a makeshift bomb near New Zealand's defence base at Do Abe in Bamiyan province on Sunday.

Baker was New Zealand's first female combat casualty since Vietnam. The deaths came only two weeks after an insurgent attack claimed the lives of two other New Zealand soldiers, lance corporals Rory Malone and Pralli Durrer, both 26, on August 5.

Key said the bodies of the three soldiers would likely return to New Zealand on Thursday, and the memorial would probably be held on Saturday.

Details of the service were not yet available.

He has already confirmed he would attend the service, even if it clashed with the Pacific Forum which starts on Monday.

The prime minister had been criticised for not being at Malone and Durrer's memorial because his son was playing baseball in the United States.

The latest deaths has taken the number of Kiwi soldiers who have died in Afghanistan to 10.

EARLY DEPARTURE FOR TROOPS

Cabinet wanted to pull the 140-troop Provincial Reconstruction Team (PRT) out in April, but Key said the decision to move the departure date from September had not been influenced by the recent deaths.

The early withdrawal was largely due to the Japanese beginning renovations at the Bamiyan airport in May, Key said.

"That renders that airport unusable by our PRT for at least about six months.

"The issue we went to the Japanese with is to say 'look if you can push back that date, then in theory we could stay in Bamiyan a bit longer, but if you don't, the only option that we would have is to take all our people by road out to Kabul to get them out of Afghanistan."

However, the road to Kabul was too dangerous and getting the soldiers out that way was "unacceptable," Key said.

"The Japanese have come back to us and said that they are not terribly flexible around that date. That's largely the reason why we are almost certainly going to leave in April."

Key was today asked if the Defence Force would stop patrolling the dangerous north-east border of Bamiyan to protect New Zealand lives. A Hungarian PRT in neighbouring Baghlan does not patrol that province's most dangerous area to protect its troops.

"That is a legitimate question and the sort of thing we discuss with our military personnel," he said.

However, the prime minister said not patrolling the area could make it more dangerous for New Zealand troops along the road to the Bamiyan township where the main Kiwi base is located.

"As part of the withdrawal from Afghanistan the northern bases (Romero and Do Abe) would have to be closed earlier than the main Kiwi base," he told TV3.

Any retaliatory strike against the insurgents and the bomb-maker who created the improvised explosive device (IED) which killed the three troops would be handled by another nation's special forces, most likely the United States, Key said.

The Government had ruled out sending the SAS back into Afghanistan in a combat role but has said a group of SAS logistics troops could return to help gather intelligence.

Key said there was not yet enough information about the insurgents for a strike.

"We've had pieces of information but not enough to put the whole puzzle together."

There had been no discussions about bringing home the current rotation of troops early. They were due to leave in October.

Labour leader David Shearer said he was happy with an April withdrawal of New Zealand's troops from Afghanistan, but he called on the Government to set a date and stick with it.

"New Zealanders want to see a fixed point at which we are going to be pulling our people out. The Government needs to make sure that is the date and then they should work towards it," he said on his way to the weekly caucus meeting of Labour MPs.

Defence Force chief Lieutenant General Rhys Jones said yesterday the contingent had been hit hard by the deaths and were receiving specialist psychological support.

Key said he hadn't spoken to the three troop's families but planned to visit them either with Jones or separately.

PULLOUT CAN'T BE RUSHED - DEFENCE EXPERT

Victoria University professor of strategic studies, Robert Ayson, said there were logistical and strategic reasons the Government would not bring them home any earlier than April.

"You can't do it in a month or six weeks," Dr Ayson said.

"And if they really accelerated it, the risk is they then come across as responding to the timetable the insurgents are setting."

The Taleban has claimed responsibility for the attack, which follows reports that Taleban leader Mullah Mohammed Omar warned coalition allies their troops would be targeted.

In a statement last week, the Taleban leader reportedly said it was "a pity that some countries have become a scapegoat for the interests of America" and called on the countries' citizens to "prevent your governments from doing this".

Next year marks a decade since New Zealand troops were sent to Bamiyan and Dr Ayson said they had improved the situation.

"It's a better place than it would have been if they hadn't been there, that's almost certain. The national police are better trained than they would have been . . . the governor is more secure than she would have been . . . but are they leaving at a time when the local security forces are able to take on those roles and provide security for the population of Bamiyan? I don't know."

Even if New Zealand troops stayed another five or 10 years, local forces would probably not be ready, he said.

'WE'VE SERVED LONG ENOUGH'

University of Canterbury political science professor Alex Tan said deaths were "a heavy hit" for a proportionally small New Zealand reconstruction team.

He said the casualties would be "an important variable" in the Government's decision-making over withdrawing forces, despite Key saying this was not the case.

"Casualty is an important variable in the consideration of any government," Tan said.

"If you only have 30, 40, 50 of us over there, that's 10 per cent."

Tan said New Zealand had not shied away from its collective responsibility as a United Nations member and "we have paid it in blood".

The Government would follow the lead of other countries in handing the responsibility for the reconstruction to the Afghan Government, he said. "[United States President] Barack Obama has been saying we are ready to pull out. New Zealand and the world community cannot be there forever," he said.

"We've been there since 2003. From the Government's perspective, we're a small country and we've been participating long enough."

Tan said the Taleban may have targeted the soldiers because of the success of their reconstruction efforts in Bamiyan.

"We are in an area where we have actually changed the lives of local people."